1. Field of the Invention
The present invention relates generally to optical access networks, and more particularly to an optical network unit for use in optical access networks.
2. Description of the Related Art
In an optical subscriber access network, the use of single-mode optical fiber is contemplated as a transmission medium between a central office and subscribers. A number of topologies such as FTTH, FTTC and FTTB have been proposed for an optical network unit (ONU) for terminating optical fibers. The FTTH (fiber-to-the-home) is one in which the optical network unit is located in a subscriber premises, the FTTC (fiber-to-the-curb) is one in which the optical network unit is located in a curb, and with the FTTB (fiber-to-the-building) the ONU is located within a building or an apartment house. With the FTTC and FTTB topologies, the connections between the ONU and a terminating equipment at subscriber premises are called drop lines which are currently contemplated as comprising twisted pairs of copper wires or a coaxial cable. A proposed future access network for broadband ISDN employs single-mode optical fibers for application to drop lines.
However, the installation of a single-mode optical fiber as a drop line between a subscriber and an optical network unit is a costly investment, particularly its optical interface. This would represent an obstacle to the deployment of broadband optical networks and services.
While the single-mode optical fiber operates at wavelengths outside of the visible spectrum, plastic and plastic-clad fibers operate in the visible spectrum. Since the visible spectrum operation permits the use of conventional microelectronic technologies for implementing optical interface, the optical interface of the plastic and plastic-clad optical fibers is much less costly than that of the interface of the single-mode optical fiber, although the transmission medium cost of the plastic and plastic-clad fibers is comparable to that of the single-mode fiber. Additionally, multimode optical fiber is much less expensive than single-mode optical fiber.